This invention relates to an improved wide-band amplifier adapted to drive a capacitance load linearly over a broad range of frequencies.
In an oscilloscope using an electrostatic deflection type cathode-ray tube, the vertical deflection is obtained by applying an input signal between vertical deflection electrodes after amplifying it with fidelity. To cope with the increase of the frequency range covered in oscilloscope observation, a wide-band linear amplifier is usually used for the vertical deflection purpose.
Usually, a Class A wide-band amplifier is used as a vertical deflection amplifier. Since a considerable electrostatic capacitance is present between the vertical deflection electrodes, the load impedance of the vertical deflection amplifier of this class reduces with increasing input signal frequencies. For this reason, the vertical deflection amplifier requires considerable output power for a sufficient driving voltage of high frequencies.
In order to permit considerable output power and nevertheless insure a satisfactory linearity, the vertical deflection amplifier is made to carry a large bias current, and the operating point is set to the optimum point in the input/output characteristic.
However, when the input signal amplitude is low though the signal frequency may be high or when the input signal frequency is low so that the load impedance is high, substantially no large bias current is required. Since the vertical deflection amplifier carries a large current even in such cases, its power consumption is increased correspondingly, which is undesired in view of economy. Particularly, in a portable oscilloscope the battery power supply is soon used up due to high power consumption by the vertical deflection amplifier.
Attempts have been made to cope with this drawback; for example there have been proposed a "Self-adaptable Amplifier" (disclosed in Japanese Patent Publication No. 7403/1972) and "Amplifier Circuit" (disclosed in Japanese Patent Publication No. 23322/1981). These circuits, however, have complicated constructions and cannot be produced inexpensively.